Like all testing methods, ultrasound inspection is also standardized and is carried out in accordance with guidelines, such as according to DIN EN 10228-3 1998-07 Zerstörungsfreie Prüfung von Schmiedestücken aus Stahl—Teil 3: Ultraschallprüfung von Schmiedestücken aus ferritischem and martensitischem Stahl (Non-destructive testing of steel forgings—Part 3: Ultrasonic testing of ferritic or martensitic steel forgings), which is hereby incorporated by reference. Suitable testing devices and methods are known for the non-destructive testing of a workpiece by means of ultrasound. General reference is made to the textbook by J. and. H. Krautkrämer, Werkstoffprüfung mit Ultraschall, ISBN-13: 978-3-540-15754-0, 5th edition (1986), Springer (Berlin).
These methods are generally based on the reflection of sound on boundary surfaces. The sound source most frequently used is an ultrasonic probe or probe whose radiation is in the frequency range of 10 kHz to 100 MHz. In the case of the pulse-echo method, the ultrasonic probe does not emit a continuous radiation, but very short sound pulses with a duration of 1 μs and less. The pulse emanating from the transmitter passes through the workpiece to be inspected with the respective speed of sound, and is almost completely reflected at the solid-air boundary surface. The sound probe is most frequently not only able to transmit pulses, but also to convert incoming pulses into electrical measuring signals; it thus also works as a receiver. The time required by the sound pulse to travel from the transmitter through the workpiece and back again is measured with an oscilloscope or a computer unit as an evaluation unit. Given a known speed of sound c in the material, the thickness of a workpiece, for example, can thus be checked. The core of such a probe is at least one ultrasonic transducer, e.g. in the form of a piezo-electric element. Furthermore, it is known, for example from WO 2007/144271, to use a phased array of several, separately controllable ultrasonic transducers that are in a fixed spatial relation for the generation and reception of the ultrasonic pulses.
The transducer(s) are most frequently coupled to the workpiece to be inspected with a matching layer—also referred to as leading body—disposed therebetween and having, for example, a wedge shape and most frequently made of a thermoplastic synthetic material such as poly(methyl methacrylate) (PMMA). A coupling surface is provided on the leading body via which the sound generated by the ultrasonic transducer(s) can be coupled into the workpiece to be inspected, with the wedge shape causing the sound to enter into the workpiece obliquely. The leading body and the piezo-electric element (s) are generally disposed in a housing which is closed on its one side and which, on its other side, has a coupling aperture through which the ultrasound emitted by the sound coupling surface can exit.
In order to couple the workpiece and the probe, i.e. the leading body, a couplant (e.g. a glue (solution), gel, water or oil) is applied onto the surface of the workpiece to be inspected. The surface to be tested is most frequently passed over with the probe. This can take place manually, mechanically or automatically (for example in production lines). In the latter case, the test piece is often immersed in a suitable liquid (immersion technique) or wetted in a defined manner for the purpose of transmitting the sound signal.
The knowledge of the dimensions of the leading body that are relevant for the ultrasound inspection is, for example, essential for exactly locating flaws of a workpiece and/or for determining the dimensions of the workpiece. Furthermore, the geometry of the leading body has a strong influence on ultrasound inspections using the so-called DGS method, in particular if the latter is carried out with a probe that permits an electronic adjustment of the insonification angle. In the case of a wedge-shaped leading body, the dimensions relevant for the ultrasound inspection are, for example, the wedge angle alpha and the distance d between the coupling surface (i.e. the boundary surface contiguous to the workpiece to be inspected) of the leading body and the center of the surface covered by the transducers on the opposite boundary surface of the leading body.
It is known to configure the connection between the ultrasonic transducers and the leading body so as to be detachable, in order to be able to vary the inspection conditions, such as, for example, the insonification angle, during the ultrasound inspection of the workpiece, in accordance with the workpiece geometry to be inspected and/or the desired inspection direction. Moreover, it is known to store the specific data of the leading wedge in a non-volatile memory connected to the leading wedge and the read them out during the ultrasound inspection and transmit them to the evaluation unit. This is known, for example, from DE 3327526 A1, but proves to require a lot of effort in practice because data communication between the leading body and the evaluation unit is required. Moreover, faulty mounting of the leading body on the ultrasonic transducer (e.g. twisted direction of mounting) also cannot be detected in this manner.
Moreover, it was found that the dimensions of the leading body changes due to wear, which can be ascribed, among other things, to the frequently used thermoplastic, and thus often very soft, synthetic materials and the usually manual displacement of the leading body over a surface of the workpiece to be inspected. Consequently, a check of the dimensions of the leading body that are relevant for the ultrasound inspection is required. Basically, checking the dimensions of the leading body is known from DE 3327526 A1. However, this is done by means of a back-face echo of a special calibration body that must be disposed adjacent to the coupling surface of the leading body. This is comparatively complex and the measurement is adversely affected with regard to its accuracy by the variation of the calibration body and the coupling between the calibration body and the leading body.
Furthermore, the determination of the thickness in a workpiece using the back-face echo of the workpiece is known from DE 3441894 A1.